1. Field of the Invention
The present invention relates to push switches employed as input units in a range of electronic devices, and more particularly to two-step push switches in which a first switch operates by a first push and a second switch operates by a further push.
2. Background Art
With electronic devices becoming increasingly smaller, components are also more densely packed inside. Push switches with two-step tactile feedback, which are employed in input units of these electronic devices, also need to become smaller and slimmer to save mounting space.
A conventional push switch with two-step tactile feedback is described next with reference to FIGS. 8 to 14.
FIG. 8 is an outline view of the conventional push switch. FIG. 9 is a sectional view taken along line 9-9 in FIG. 8, and FIG. 10 is a sectional view taken along line 10-10 in FIG. 8. FIG. 11 is a plan view of the conventional switch case. FIG. 12 is a sectional view taken along line 9-9 in FIG. 8, illustrating the operation of a first step. FIG. 13 is a sectional view taken along line 9-9 in FIG. 8, illustrating the operation of a second step. FIG. 14 is a chart of tactile curves for the conventional push switch.
In FIGS. 8 to 11, switch case 1 is made of insulating resin, and has recess 1A that has an open top. Switch case 1 also has movable contact housing recess 1B on the inner bottom center of this recess 1A. Central fixed contact 2 is disposed at the center of this movable contact housing recess 1B, and peripheral fixed contact 3 is disposed at two points symmetrical about central fixed contact 2. Outer fixed contact 4 is disposed at two points symmetrical about central fixed contact 2, outside movable contact housing recess 1B.
Central fixed contact 2 is electrically connected to third connecting terminal 5, and peripheral fixed contacts 3 are electrically connected to second connecting terminal 6. Outer fixed contacts 4 are electrically connected to first connecting terminals 7.
Dome-shaped second movable contact 8 is disposed on movable contact housing recess 1B at the inner bottom center of recess 1A of this switch case 1. The bottom edge of the outer periphery of this second movable contact 8 contacts peripheral fixed contacts 3. The center of this second movable contact 8 faces central fixed contact 2.
First movable contact 9 includes ring portion 9A, narrow central portion 9B at the center which is bridged to ring portion 9A by a coupling bar dividing the space inside ring portion 9A into two parts, and peripheral portion 9C provided on an outer periphery of ring portion 9A at opposing positions. A draw piece expanding upward is provided along the circumference at equal intervals of 90°. This first movable contact 9 is disposed on outer fixed contact 4 by its peripheral portion 9C. In this state, central portion 9B is positioned over second movable contact 8 at a predetermined distance. Projection 9D extending downward is provided at the center of central portion 9B.
Vertically movable operating member 10 is disposed on the top face of central portion 9B of first movable contact 9.
In addition, cover 11 is attached to switch case 1 so as to cover the top face of recess 1A. Operating area 10A of operating member 10 protrudes upward from central hole 11A in cover 11.
The conventional push switch as described above is configured such that second movable contact 8 and first movable contact 9 are housed inside recess 1A of switch case 1, and operating member 10 is provided over this structure.
When operating area 10A of operating member 10 is pressed in the conventional push switch as configured above, the coupling bar, connecting central portion 9B to ring portion 9A in first movable contact 9 underneath, inverts and ring portion 9A resiliently deforms. This produces first-step tactile feedback. Projection 9D on the bottom face of central portion 9B then contacts the top center of second movable contact underneath. This establishes an electrical connection between first connecting terminal 7 and second connecting terminal 6 via first movable contact 9 and second movable contact 8.
When operating area 10A of operating member 10 is further pressed, projection 9D on central portion 9B of first movable contact 9 presses the top center of second movable contact 8. When this pressing force exceeds a predetermined level, a second-step tactile feedback is produced by the resilient inversion of a dome portion of second movable contact 8. The bottom center of second movable contact 8 then contacts central fixed contact 2. This establishes an electrical connection among first connecting terminal 7, second connecting terminal 6, and third connecting terminal 5.
When the pressing force on operating area 10A of operating member 10 is released, the dome portion of second movable contact 8, which has resiliently inverted, reverts by itself, providing tactile feedback. Accordingly, the top center of this dome portion pushes back projection 9D on central portion 9B upward, and thus its bottom face separates from central fixed contact 2. Third connecting terminal 5 therefore becomes electrically independent from first connecting terminal 7 and second connecting terminal 6.
Ring portion 9A of first movable contact 9 and the coupling bar connecting ring portion 9A to central portion 9B then reverts by itself, providing tactile feedback. This makes projection 9D of central portion 9B separate from the top face of second movable contact 8. First connecting terminal 7 and second connecting terminal 6 thus also become electrically independent. Accordingly, the push switch returns to its original state without any pressing force, as shown in FIGS. 8 to 10.
One prior art related to the present invention is disclosed in Japanese Patent Unexamined Publication No. 2004-031171.
In this conventional push switch, the first-step tactile feedback is produced when central portion 9B of first movable contact 9 is pressed by a pressing force, and the draw piece of ring portion 9A is resiliently deformed. Then, the second-step tactile feedback is produced when projection 9D on central portion 9B of first movable contact 9 presses the center of second movable contact 8 by further pressing central portion 9B, and second movable contact 8 is resiliently deformed.
These operational changes are described using a chart of tactile curves in FIG. 14 in which a pressing load is plotted along the vertical axis and the distance is plotted along the horizontal axis.
Tactile curve 14 in FIG. 14 shows the operational changes of independent first movable contact 9. In this tactile curve 14, a difference between maximal value 14A of the operation force and a minimal value 14B of the operation force produces tactile feedback. If this difference is large relative to the pressing load at maximal value 14A, the user feels strong tactile feedback. The distance between these points affects the crispness of the feedback. When the distance between maximal value 14A and minimal value 14B is long, tactile feedback is produced slowly. This first movable contact 9 is designed to allow further resilient deformation because it needs to press second movable contact 8 after passing minimal value 14B, where the first-step tactile feedback is produced.
Next, operational changes of independent second movable contact 8 are shown in tactile curve 15 in FIG. 14. The dome portion of second movable contact 8 resiliently inverts and produces the tactile feedback between maximal value 15A and minimal value 15B. Then, second movable contact 8 does not move further and only the pressing load increases because the dome center on the bottom face of this second movable contact 8 touches central fixed contact 2 after the dome portion is resiliently inverted.
The tactile curve of the conventional push switch is achievable by combining tactile curves 14 and 15 in FIG. 14. This is indicated as tactile curve 16.
In tactile curve 16, the tactile curve for first movable contact 9, which is the first step, changes in the same way as tactile curve 14, but then first movable contact 9 is further deformed while second movable contact 8 is deformed after the first-step tactile feedback is produced. This means the two movable contacts are pressed simultaneously.
In other words, at maximal value 16C and minimal value 16D in FIG. 14, which is the second-step tactile feedback, the pressing load of first movable contact 9 corresponding to its operating position (distance) is applied in addition to the pressing load of second movable contact 8 in the tactile curve. This makes it complicated to achieve the intended pressing load. In particular, the load for further deforming first movable contact 9 after passing its minimal value 14B increases in a quadratic curve. Accordingly, the pressing load of minimal value 16D in this tactile curve 16 further increases, and the difference between maximal value 16C and minimal value 16D shrinks, resulting in dull tactile feedback for the second step.